The present invention generally relates to a solventless, thermal degreasing process and, more particularly, it relates to solventless, thermal degreasing of aluminum-based sheets or parts in a reactive gaseous atmosphere and subsequent brazing of the cleaned aluminum-based sheets or parts.
Degreasing of metal is well established in the art and has been used extensively in processing metallic scrap material. Representative of the prior art patents relating to the degreasing of metallic scrap are U.S. Pat. Nos. to Kennedy (4,508,564), Fitzpatrick (4,654,088), Ruthven (2,104,102), Mathis (3,650,830), Erman (3,627,289), Ripoche (2,595,411), and Stephens (4,010,935).
However, none of these references disclose a method whereby degreasing takes place in a solventless atmosphere. For example, Ruthven (2,104,102) discloses a thermal degreasing method which involves dipping a heated article in a solvent such as trichloroethylene at such a rate and in such quantity that the article will obtain a temperature of at least the vapor temperature of the solvent after the article leaves the bath of boiling solvent so that the article is devoid of a film of solvent as well as of greasy material when it leaves the vapor atmosphere. The use of solvents, such as trichloroethylene, for the reactive atmosphere during degreasing is an expense and may pose environmental concerns.
In addition, a problem which has occurred with degreasing methods at elevated temperatures especially when using aluminum-based parts has been the further oxidation of the underlying aluminum alloy surface or the complete destruction of the protective oxide layer which normally exists. U.S. Pat. No. (2,856,333) Topelian and U.S. Pat. No. (4,684,411) Johnson et al both specifically disclose oxidizing the underlying aluminum-based alloy as part of their thermal degreasing method. By increasing the thickness of the initial thin, protective oxide layer of aluminum-based alloys in this manner, subsequent brazeability of the metallic alloy is severely compromised. That is, the additional oxidation, that can occur in high temperature thermal degreasing (i.e., around 500.degree. C. and above) forms a layer which is not readily brazeable. With magnesium bearing alloys, oxidation produces a duplex oxide on the outermost surface which degrades brazability. Careful maintenance of the temperature during thermal degreasing is necessary in order to prevent disruption of the existant protective aluminum oxide layer and thereby preserve the brazability of the aluminum-based sheet or part.
In this regard, U.S. Pat. No. (4,016,003) Stauffer discloses the careful maintenance of temperatures to avoid ignition of volatile materials which could also conceivably preserve the underlying protective oxide layer of the metallic alloy in cleaning aluminum scrap. However, Stauffer does not control the temperature for this purpose but rather for the purpose of avoiding the ignition of volatiles. Moreover, Stauffer does not subsequently braze the metallic alloy. Rather, Stauffer subsequently melts the alloy as part of the scrap recycling process which clearly destroys the underlying protective oxide layer.
Another degreasing method is shown by Hetherington in "ULVAC Aluminum Brazing Furnaces" Supp. to ULVAC Sales brochure E 3009 "Aluminum Vacuum Brazing Furnace" FB Series, which discloses a degreasing method that has been implemented in the Japanese automotive industry. Hetherington's solventless degreasing method for aluminum based alloys prior to brazing requires drawing a vacuum which precludes oil and grease from completely reacting. This method depends upon the complete evaporation of surface contaminants for surface cleaning. Dissociation of the surface contaminant during evaporation can produce residual surface contaminant species. Moreover, because vacuum conditions are required, Hetherington's degreasing method is not as economical as is desirable.
Accordingly, there remains a need in the art for a degreasing method for aluminum-based alloys which preserves the underlying protective oxide layer so as not to compromise any subsequent brazing and which removes surface contaminants which can not be completely removed by evaporation alone. Further, the degreasing method should be solventless so to reduce cost and do as not to pose any adverse toxicological and/or environmental problems. Also, there is a need for a degreasing method which does not require a drawing a vacuum.